1. Field of the Invention
The invention relates to color shutters developed using liquid crystal (LC) active elements, and more particularly to a color shutter using a reflective polarizer.
2. Description of the Related Art
Conventional projection displays have used different systems for providing each color of red, blue and green. In many variations, separate CRTs were used. In other variations, separate transmissive liquid crystal panels were used. In either case, three separate systems were required, which greatly increased the cost of the projection display. Recently there have been efforts using liquid crystal panels and DMDs, as developed by Texas Instruments, to reduce cost by using a single device instead of three separate devices. This fundamental change from spatial to sequential operation resulted in a reduced cost of the overall projection display. However, problems related to developing the sequential color fields were created. White light had to be sequentially provided from a lamp to the imager device as red, green and blue light. Some form of color wheel or color shutter was used for this purpose. Conceptually the color wheel contained three transmissive sections, i.e., red, green, and blue. As the wheel rotated, each color of light was passed for a time by the filters in the wheel.
However, the color wheel is not a preferred solution in many cases, because of the physical size needed to provide a reasonable time for each sequential color. Liquid crystal-based color shutters would be preferred, as they are typically small. Liquid crystal color shutters, however, use absorptive polarizers in their operation. Generally, the LC color shutters use some form of polarization of the different colors of light combined with a rotation by the LCs. The rotated light is then absorbed by various polarizers. The light energy must be absorbed by the polarizers to provide the desired effect. This is acceptable in the original uses in color cameras, but becomes a problem for high power lamps used for projection displays. To get sufficient light to the screen, in either front or rear projector applications, a very large amount of light must be provided from the lamp. The various losses in the system, which may approach 99%, reduce the light levels to the final projected level. As a result, the light level at the color shutter, which is placed relatively early in the light path, is very high. Thus, the absorptive polarizers would be required to absorb large amounts of light, much more than they could reasonably bear. The color shutters would either fail or breakdown in the short term or degrade in the long term due to improper thermal effects. Therefore, LC color shutters are not a reasonable alternative to the color wheel. The designer is left without a good solution for the single imager, sequential projection display system.